In-place real-to-complex FFTs?

[jipolanco]

Not that simple due to the change from a real to a complex element type.

This will require changes to ManyPencilArray in PencilArrays, to allow for variable element types of the aliased arrays. I'm not sure if this can be done efficiently with reinterpret, and unsafe workarounds may be needed.

Also, in-place r2c FFTs are not currently wrapped by FFTW.jl (due to this and other complications), and custom wrappers will likely be needed.